G1942

Ash Rust

Ash rust can disfigure or prematurely defoliate ash trees. This NebGuide discusses ash rust symptoms, disease cycle and management.

• Symptoms
• Disease Cycle
• Management

Ash rust is distributed throughout most of eastern North America and can be seen in Nebraska sporadically. It is caused by the fungus Puccinia sparganioides, and can infect several species of ash (Fraxinus) including green, white, and black.

The alternate hosts for this rust include several species of cordgrass (Spartina spp.) and a marsh grass (Distichlis spicata). This disease is more prevalent and severe in areas where the alternate hosts (cordgrass species and marsh grass) are most abundant, such as wet or saline soils.

Symptoms

Ash rust is commonly found on leaves, though infection can occur on all the current year’s growth. Initial symptoms are yellow to orange spots on the upper leaf surface and chlorotic spots on twigs and petioles. Approximately two weeks following infection, bright orange lesions (1/6 to 1/2 inch diameter) appear on petioles, stems, and on the lower side of the leaf (Figure 1).

These lesions contain two spore types. Spermatiospores or pycniospores appear first but they generally are not noticed. Then aeciospores become visible (Figure 2). Diseased tissue may swell, causing distortion of leaves, sharp bends in the petioles (Figure 3) and roughly egg-shaped galls on twigs and seeds.

If infections are severe, premature defoliation of the tree will occur. Trees with severe infection can also suffer deformation or death of the current year’s shoot. If that shoot is a leader, its death can result in a multi-stemmed tree.

There are no other rusts that affect ash; however there are several rust species that infect cordgrass. The rust species that infect cordgrass are difficult to distinguish. It is important to note that the presence of rust on cordgrass does not necessarily mean ash trees in these surrounding areas are threatened. Rusts observed on cordgrass include Puccinia distichlidis, P. seymouriana, and Uromyces acuminatus. Alternate hosts of P. distichlidis include fringed loosestrife (Lysimachia ciliate) and milkwort (Glaux maritima). Alternate hosts for P. seymourina include buttonbush (Cephalanthus sp.), dogbane (Apocynum sp.), and milkweed (Ascelpias sp.).

Figure 1. Leaves of ash infected with ash rust. The orange structures on the lower surface of leaves are the aecial cups in lesions.		Figure 2. Closeup of distinctive pycnida and aecial “cluster” cups with orange-yellow aeciospores proliferating from the surface of infected tissue (10x magnification).
Figure 3. Ash leaf rachis (leaf stem) distorted by ash rust.		Figure 4. 200x magnification of teliospores produced on cordgrass.

Disease Cycle

Puccina sparganioides survives the winter on cordgrass and marsh grass in structures called telia. This fungus does not overwinter on ash. During warm, wet springs the teliospores (Figure 4) within the telia will germinate to produce basidiospores. Ash trees become infected when moist air moves the basidiospores onto wet leaves, petioles, and green shoots where these spores germinate and infect the plant tissue.

As the fungus develops in the ash tissue, it produces spore-producing structures called spermagonia or pycnia in the yellow spots on the upper leaf surface and on chlorotic spots on petioles and stems. The bright orange lesions containing aeciospores (Figure 2) appear on the lower leaf surface, petioles and stems approximately two weeks after the spermagonia appear

Management

Typically, ash rust disfigures and defoliates trees but does not seriously threaten the health of the tree. Repeated ash rust infections can weaken trees, especially those grown near wetlands where cordgrass and marsh grass grow. Weakened trees are more susceptible to winter damage and winter dieback. If the leader of a small tree dies from infection, the tree should be pruned the following year to train the stem to a single leader. Prune in late February or early March when the tree is still dormant and prior to the active growth.

Valuable trees can be protected from severe infections with timely fungicide treatments (Table I). During wet springs, begin the fungicide applications as the buds break open and repeat according to label instructions until the foliage is fully developed. The use of fungicides that have systemic properties (i.e. thiophanate-methyl) will have longer residual periods, thus fewer reapplications will be required.

This publication has been peer reviewed.